Scr firing circuit

ABSTRACT

An SCR firing circuit is provided for each SCR in a multiphase power circuit supplying a DC motor. Each firing circuit includes an output transistor, controlled in the on-off mode by a multivibrator circuit, for providing continuous gate drive to its associated SCR when the multivibrator switching circuit turns it on. A charging circuit includes a capacitor and a transistor connected so that a control signal applied to the transistor determines the rate of capacitor charge. When a predetermined voltage is applied across the capacitor, a unijunction transistor fires and applies a trigger signal to the multivibrator switching circuit for applying a continuous gate drive to the SCR. A signal from the input AC line which supplies the power circuit is utilized to reset the multivibrator switching circuit and remove the gate drive at the proper time.

United States Patent 72] inventors Dudley D. Nye, Jr.

Fort Lauderdale; Thomas Pantelakis, Margate; Donald J. Wilson, FortLauderdale, all of, Flat. [2]] Appl. No. 838,977 [22] Filed July 3, 1969[45] Patented June 15, 1971 [73] Assignee Borg-Warner CorporationChicago, Ill.

[54] SCR FIRING CIRCUIT 11 Claims, 2 Drawing Figs.

[52] US. Cl 307/247, 307/252, 318/138, 318/345 [51] Int. Cl H03k 17/00[50] Field of Search 307/247, 252, 70, 71, 72, 90; 318/] 38, 345

[56] References Cited UNITED STATES PATENTS 3,264,626 8/1966 Mounce307/252 X 3,426,969 2/1969 Anderson, Jr. 307/247 X Primary Examiner.lohnS. l-leyman Assistant Examiner-John Zazworsky An0rneys- Donald W.Banner, William S. McCurry and John W, Butcher ABSTRACT: An SCR firingcircuit is provided for each SCR in a multiphase power circuit supplyinga DC motor. Each firing circuit includes an output transistor,controlled in the onoff mode by a multivibrator circuit, for providingcontinuous gate drive to its associated SCR when the multivibratorswitching circuit turns it on. A charging circuit includes a capacitorand a transistor connected so that a control signal applied to thetransistor determines the rate of capacitor charge. When a predeterminedvoltage is applied across the capacitor, a unijunction transistor firesand applies a trigger signal to the multivibrator switching circuit forapplying a continuous gate drive to the SCR. A signal from the input ACline which supplies the power circuit is utilized to reset themultivibrator switching circuit and remove the gate drive at the propertime.

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. s a; a +1 I I n s v (Q f I I m Inventors 7 Dudley D. Nye,dr. L Thomas*P'onfelokis Donald J. Wilson dqnug/ J I A110 ny SCR FIRING CIRCUITBACKGROUND OF THE INVENTION Various arrangements for regulatingconduction and nonconduction of semiconductor power switches, such assilicon controlled rectifiers (SCRs), have been provided to regulate theamount of electrical power transferred to an electrical motor. In thisway the 'motor speed can be determined by a control circuit or firingcircuit which regulates the on and off times of the SCRs. When aconventional single-phase or three-phase power main supplies the circuitin which an SCR is connected, a sudden power outage or notch" in theinput voltage waveform can cause a particular SCR to be switched offbefore the proper time dictated by the firing circuit. The inadvertentlyturned off SCR will then remain off until it is again switched on by thefiring circuit in the next cycle of operation.

One of the reasons this accidental turnoff occurs is that a snubber"circuit is frequently coupled across the armature circuit of the motorto minimize effects of the transient voltages that appear across thearmature when the SCRs are first turned on. The snubber circuitconventionally includes a capacitor series-coupled with a resistorhaving a relatively low ohmic value. Thus as the armature voltage buildsup, if there is a sudden notch or reduction in the amplitude of thesupply voltage, the voltage then appearing across the capacitor in thesnubber circuit can back bias the conducting SCR so as to commutate itofi before its appointed turnoff time. However if a continuous gatedrive signal were applied to the SCR its conduction would be restored assoon as the notch or voltage decrease was past. The general practice inthis art has been to apply the SCR gating pulse over a pulsetransformer, and thus the gate drive is only maintained for a briefperiod to insure that the SCR is turned on at the proper time.

It is therefore a primary consideration of the invention to provide anSCR firing circuit which applies a'continuous gate drive pulse to theSCR which should be conductive, and maintains this continuous gate drivesignal over the entire time period during which the SCR should bemaintained conductive.

Another consideration of this invention is to provide an optionalinterconnection between the three firing circuits for a three-phase,three SCR, three diode, rectifier bridge, to insure a continuous firingpulse which is inhibited during commutation if the SCR anode goesnegative before thepower line synchronizing or reset signal is received.

SUMMARY OF THE INVENTION This invention includes a firing circuit for anSCR which has an anode, a cathode, and a gate. The firing circuitcomprises a semiconductor output switch connected to provide acontinuous gate drive signal to the SCR gate when the semiconductoroutput switch is conducting. A switching circuit is connected to enableand disable the semiconductor output switch, to provide and remove thecontinuous gate drive signal. A charging circuit includes a capacitorand a variable semiconductor switch, connected such that the capacitoris charged at a rate which is a function of the conduction level of thevariable semiconductor switch. Means is connected to operate, inresponse to a predetennined voltage appearing across the capacitor, toapply a trigger signal to the switching circuit and thus provide thecontinuous gate drive signal. A reset means is connected to effect rapiddischarge of the capacitor, and to disable the semiconductor outputswitch for removing the gate drive signal.

THE DRAWINGS In the several figures of the drawings like referencenumerals identify like elements, and in the drawings:

FIG. 1 is a schematic diagram, partly in block form, depicting thepresent invention incorporated in a motor energizing system; and

FIG. 2 is a schematic diagram of a firing circuit shown generally inFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION FIG. I depicts a motor-energizingsystem in which the armature circuit of a motor 20 receives energy froma power circuit 21 which, in turn, is supplied with three-phase ACenergy over three input conductors 22, 23 and 24. The level of energypassed to the motor is determined by the conduetion times ofsemiconductor power switches or silicon controlled rectifiers (SCR's)25, 26 and 27 which, in turn, are regulated by gating signals providedby firing circuits 28, 29 and 30. The firing circuits are controlled byan output signal received over circuit 31 from a control amplifier 32,which operates in response to different input signals, and are alsocontrolled by synchronized reset signals received over conductors 71-73and transformer 74. 7

Motor 20 is energized as DC energy is passed through power circuit 21and applied between conductors 33 and 34. When switches 35 and 36. areclosed, current flows through motor 20 in a first direction and effectsmotor rotation in a given angular direction. When switches 35, 36 areopened and switches 37, 38 are closed (by control components notillustrated because they are well known), current flows through motor 20in the opposite direction and effects motor rotation in the oppositeangular direction. A snubber circuit, including a capacitor 55 coupledin series with a resistor 56, is coupled in parallel with the armatureof motor 20 to minimize the effects of transients when the SCRs arefirst turned on.

Motor field winding 40 is coupled between conductors 22 and 33. Threediodes 41, 42 and 43 are respectively coupled in series with thesemiconductor switches 25, 26 and 27. The

input conductors 22-24 are respectively connected to the commonconnections between each diode-SCR pair.

Although the semiconductor power switches 25-27 are illustrated assilicon controlled rectifiers, other components such as thyratrons,ignitrons, power transistors, transistors, electron-discharge devicesand similar switching units can be used in their place. In anothermethod, only one semiconductor power switch is required to regulate thelevel of motor energization. Such switch can be coupled in a seriescircuit connection, in a chopper or DC-to-DC converter arrangement, inlieu of the illustrated three-phase rectifier arrangement. Siliconcontrolled rectifier 25 receives gating or turn-on signals overconductors 44, 45 from firing circuit 28; SCR 26 receives gating signalsover conductors 46 and 47 from firing circuit 29; and SCR 27 receivesturn-on signals over conductors 48, 49 from firing circuit 30.

A freewheeling" diode 50 is coupled between conductor 33 and circuitcommon or ground. Because of the inductive reactance of the motor, theturnoff of a given silicon controlled rectifier may terminate currentsupply while the motor tends to keep current flowing; the freewheelingdiode 50 maintains a path for the continuing current flow. A resistor 51is coupled between conductor 34 and ground. A potentiometer 52 iscoupled between conductor 34 and ground, and the movable arm is coupledover a resistor 53 and a conductor 54 to a control amplifier 32.

A pair of resistors 57, 58 are coupled in series between conductor 33and ground, and conductor 60 is coupled between the midpoint of thesetwo resistors and an input connection of control amplifier 32. Thecontrol amplifier also receives a speed reference signal over line 61from a potentiometer 63. Three diodes 64, 65 and 66 are coupled inseries between conductor 54 and ground to protect against an undulylarge signal being passed over conductor 54 to the control amplifierstage if resistor 51 were to open, or if for some other reason an excesscurrent were to be suddenly supplied over resistor 53. A resistor 67 iscoupled between ground and a common conductor 68 which is coupled to thecathodes of all the semiconductor switches 25, 26 and 27. Conductor 70is coupled to the common connection of resistor 67 and common conductor68, and is also coupled to control amplifier stage 32.

Transformer 74 has three primary windings 75, 76 and 77 coupled to theinput conductors 22, 23 and 24 over which three-phase AC energy isreceived from any conventional source. The three secondary windings 78,79 and 80 of transformer 74 have their lower terminals coupled together,and to conductor 70. The upper end of each secondary winding isrespectively connected to a diode 81, 82 and 83, the cathodes of whichare coupled in common and to an output conductor 84. A filter capacitor85 is coupled between output conductor 84 and common conductor 70. Inaddition to the respective diodes 81-83, a sync or reset conductor 71,72 and 73 is also coupled to the upper end of each of the secondarywindings 78---80.

All the firing circuits 28-30 are the same, and thus the illustrationand description of a single circuit will suffice for an understanding ofall three. In addition it is noted that the circuit and techniques ofthis invention are applicable to single phase systems, in which only asingle firing circuit is utilized.

ln firing circuit 29, depicted in FIG. 2, a charging circuit includes acapacitor 87 coupled in series with a variable semiconductor switch,shown as a PNP-type transistor 88. Also coupled in series in the samecharging circuit are a resistor 90 and a potentiometer 91, but thoseskilled in the art will appreciate that a single resistive component canfunction in place of the two shown. A diode 92 is coupled between anenergizing terminal 93 and the common connection 94 to which conductor31a, potentiometer 91 and resistor 95 are all coupled. Thus terminal 93,or the conductor coupled to terminal 94, represents means for energizingthe charging circuit such that capacitor 87 is charged at a rate whichis a function of the conduction level of variable semiconductor switch88.

Conductor 31b is coupled to the base of transistor 88, and the emitterof transistor 88 is coupled, through resistor 90 and the effectiveportion of potentiometer 91, to conductor 31a. In the illustratedportion of control amplifier stage 32, a resistor 96 is coupled betweenconductors 31a and 31b. This resistor is also coupled in series with thecollector-emitter path of a transistor 97, so that the conduction levelof this transistor, itself a function of different variables combined inthe control amplifier stage, determines the voltage drop across resistor96 and the conduction level of transistor 88.

The collector of transistor 88 is coupled to the upper plate ofcapacitor 87, to the collector of an NPN-type transistor 98, and to theemitter of a unijunction transistor 100. The base two connection of thisunijunction transistor is coupled through a resistor 101 to conductor102, and the base one connection is coupled to one side of a capacitor103 and to one side of a resistor 104. Another resistor 105 is coupledin series between capacitor 103 and the base of a first transistor 106in a multivibrator or flip-flop circuit 107. It will become apparentthat unijunction transistor 100 represents a voltageresponsive componentconnected to operate when a predetermined voltage appears acrosscapacitor 87, to apply a trigger signal through capacitor 103 andresistor 105 to the base of transistor 106 and turn this transistor on.in accordance with well-known multivibrator circuit operation, thisaction rapidly drives the other NPN-type transistor 108 off.

The emitters of transistors 106, 108 are coupled to the common outputconductor 47. The collector of transistor 106 is coupled throughcapacitor 110 to the base of transistor 108, and the collector oftransistor 108 is coupled through a resistor 111 to the base oftransistor 106. A resistor 112 is coupled in parallel with capacitor110. A resistor 113 is coupled between conductor 102 and the collectorof transistor 106. Resistor 114 is coupled between conductor 102 and thecommon connection 119 between resistor 111, resistor 115 and thecollector of transistor 108. A capacitor 116 is coupled betweenconductors 102 and 47. Circuit 107 is thus seen to be a switchingcircuit or a multivibrator-type circuit having first and secondsemiconductor switches 106, 108 connected to operate in a flip-flopmanner. Transistor 106 is gated on when a'signal is received at itsbase, and transistor 108 is driven on when it receives a signal overconductor 117 from the emitter of transistor 98. Resistor 115 is coupledbetween connection 119 and the base of a semiconductor output switch118, the collector of which is coupled through a resistor 120 toenergizing conductor 84. This output switch, shown as another NPN- typetransistor, has its emitter coupled to output conductor 46. A resistor122 is coupled between output conductors 46, 47.

Transistors 98 is connected to function as a reset semiconductor switch,with its emitter coupled to conductor 117 and its collector coupled tothe common connection between capacitor 87, the collector of transistor88 and the emitter of unijunction transistor 100. The base of transistor98 is coupled through a resistor 123 to conductor 72 over which a resetsignal is received from secondary winding 79 of transformer 74. A diode124 and a capacitor 125 are coupled in parallel, and this protectivecircuit is coupled between the base of transistor 98 and the commonoutput conductor 47.

In operation it is initially assumed that the second or reset transistor108 in the switching circuit 107 is conductive, with transistor 106 off.Conduction of transistor 108 maintains a sufficiently low voltage at thebase of semiconductor output switch 118 so that this transistor is off,and there is no gate drive supplied to SCR 26. It is additionallyassumed that capacitor 87 is just beginning to charge and has notaccumulated a voltage sufficient to fire unijunction transistor 100.Reset semiconductor switch 98 is off, and variable transistor 88 isconducting at a level determined by the signal between conductors 31aand 31b.

Capacitor 87 charges, at a rate determined by the conduction level oftransistor 88, over a circuit extending from terminal 93 and includingdiode 92, terminal 94, the effective portion of potentiometer 91,resistor 90, the emitter-collector path of transistor 88, and capacitor87 to conductor 47. When capacitor 87 accumulates a predeterminedvoltage, unijunction transistor fires and produces a pulse ortrigger'signal across resistor 104 which is coupled through capacitor103 and resistor to the base of the first transistor 106 in switchingcircuit 107.

As transistor 106 is rapidly driven on, the voltage at its collectorgoes negative toward the potential on conductor 47. This action producesa negative-going pulse which is applied through capacitor to the base ofsecond transistor 108, rapidly switching this transistor off. Astransistor 108 becomes nonconductive, the voltage at the commonconnection 119 rapidly goes positive to hold transistor 106 on viaresistor 111 and to drive the semiconductor output switch 118 intoconduction. This action completes a path for current flow from conductor84 over resistor 120, the collector-emitter path of switch 118, andresistor 122 to conductor 47. Accordingly with this arrangement,semiconductor output switch 118 remains conducting and a continuous gatedrive signal is maintained between conductors 46, 47 for applicationbetween the gate and cathode of SCR 26.

When the polarity of the voltage across secondary winding 79 oftransformer 74 reverses, a reset or positive-going turnon signal isapplied over conductor 72 and resistor 123 to the base of transistor 98,rapidly driving this transistor on. Thus transistor 98 conducts andcompletes a path for the discharge of capacitor 87 which includes thebase-emitter path of second transistor 108 in switching circuit 107.That is, the voltage at the upper plate of capacitor 87 is positive withrespect to that of its lower plate, coupled to conductor 47. Dischargecurrent flows from the upper plate of capacitor 87 over a path includingthe collector-emitter path of reset semiconductor switch 98, conductor117, and the baseemitter path of transistor 108 to conductor 47.Accordingly capacitor 87 is very rapidly discharged and transistor 108is driven on. The voltage at common connection 119 rapidly reduces toalmost zero, and this clamping of the voltage at connection 119 to thelevel of the voltage on conductor 47 turns 01? transistor I06 andtransistor 118. Thus the original circuit conditions are reestablishedand capacitor 87 again commences to charge through transistor 88 at arate determined by theoutput signal from control amplifier output stageAdjustment of potentiometer 91 provides a means .for equalizing thecharging rates in all the firing circuits,,considering that the actualcomponents in each circuit unit do not have precisely the same values asthose in the other circuits. The circuit in its reset operation can bemodified by coupling the emitter of reset transistor 98 directly to thecommon point between capacitors 125 to ,87,.to effect a more readilyapparent discharge of the capacitor 87. By connecting a resistor fromthe base of transistor 108 to the common connection 1 19 in firingcircuit 30, appropriate synchronization of the turn off in stillmaintained with the different connection of reset switch 98. In thiscase it is also necessary to similarly interconnect firing circuit 29 tocontrol firing circuit 28, and firing circuit 28 1 to control firingcircuit 30. This interconnection between the firing circuits 28-30causes the firing pulses on the SCRs to be'inhibited when the next SCRturns on, and prevents application of a positive gate signal to an SCRwhich then has a negative-going voltage applied to its anode. Thisreduces power dissipation in the SCR's.

Solely to assist those skilled in the art to make and use the inventionwith a minimum of experimentation, a table of typical circuit componentsand identifications or values is set out below. This table is given byway of illustration only and in no sense-by way of limitation. Toenergize the components, a +16 volts potential, relative to thepotential on reference conductor 47, was applied to terminal 93.Rectifier circuit 8 1.83 supplied a +15 volts potential over conductor84 to the top of resistor 120.

rosnos 2N27l4 118 2N34l7 (selected) 92,124 lN5059 87 0.22mfd.,:l0'7r,250 vdc 103 500 pfd., :lOX', 500 vdc 110 I 100 pfd., 210%,500 vdc 116 0.10 mfd.,:l0%,250 vdc 125 0.01 mfd., :101, 250 vdc 90 825ohms, :IZ'

91 750 ohms, :l0% 95,104,122 100 ohms, IIOk 101 470 ohms, :5;

105 l0Kohms,:lO% 111,112 27Ko hms,:l0% 113,114 1.8 K ohms,

115 3.9 K ohms. :5%

123 22 K ohms, 140% While only a particular embodiment of the inventionhas been shown and described, it will be apparent to those skilled inthe art that various changes and modifications may be made thereinwithout departing from the invention in its broader aspects. Thereforethe aim in the appended claims is to cover all s tch charges andmodifications as may fall within thetrue spirr'tand scope of theinvention.

What 1 claim is:

l. A firing circuit for an SCR having an anode, a cathode, and a gate,comprising:

a semiconductor output switch connected to provide a 0on tinuous gatedrive signal to the SCR gate when said semiconductor output switch isconducting,

a a switching circuit connected to enable and disable the semiconductoroutput switch to provide and remove the continuous gate drive signal,

a charging circuit, including a capacitor and a variable semiconductorswitch, connected such that the capacitor is charged at a rate which isa function of the conduction level of the variable semiconductor switch,

means connected to operate in response to a predetermined voltageappearing'across the capacitor to apply a trigger signal to theswitching circuit and thus provide the continuous gate drive signal, and

reset means connected to effect rapid discharge of the capacitor, and todisable the semiconductor output switch and remove the gate drivesignal.

2; A firing circuit as claimed in claim 1 and in which said switchingcircuit comprises first and second semiconductor switches intercoupledin a multivibrator circuit, operative to change the state of thesemiconductor output switch from nonconducting to conducting each timethe trigger signalis supplied to the switching circuit.

3. A firing circuit as claimed in claim l in which said charging circuitincludes a potentiometer, adjustable to compensate for variations in thecomponents of the firing circuit.

4. A firing circuit as claimed in claim 1 in which the means whichoperates in response to appearance of the predetermined voltage acrossthe capacitor is a unijunction transistor, and circuit means is coupledbetween the unijunc'tion transistor and the switching circuit forpassing the trigger signal to actuate the switching circuit each timethe unijunction transistor is fired.

5. A firing circuit for an SCR having an anode, a cathode,andagate,comprising:

a semiconductor output switch connected to provide a continuous gatedrive signal to the SCR gate when said semiconductor output switch isconducting,

a switching circuit having first and second semiconductor switches,connected such that-conduction of the second semiconductor switchdisables the semiconductor outputswitch and removes the gate drivesignal, and conduction of the first semiconductor switch turns off thesecond semiconductor switch and turns on the semiconductor output switchto provide the continuous gate drive signal, a charging circuit,including a capacitor, a variable semiconductor switch, and a resistivecomponent, all coupled in series with'a conductor for energizing thecharging circuit such that the capacitor is charged at a rate which is afunction of the conduction level of the variable semiconductor switch, 1

means, including a voltage-responsive component, coupled between thecharging circuit and the switching circuit, for operating in response toa predetermined voltage appearing across the capacitor to apply atrigger signal for turning on the first semiconductor switch in theswitching circuit and thus providing the continuous gate drive signal,and

means, including a reset semiconductor switch, coupled between aconductor for receiving a reset signal and the capacitor in the chargingcircuit, for effecting rapid discharge of said capacitor and conductionof the second semiconductor switch in the switching circuit to disablethe semiconductor output switch and remove the gate drive signal.

6. A firing circuit as claimed in claim 5 and in which saidsemiconductor output switch is a transistor coupled between a plane ofenergizing potential and a plane of reference potential, and a componentis coupled between the transistor and the reference potential plane toprovide the continuous gate drive signal whenever the transistor isgated on by a signal from the switching circuit.

7. A firing circuit as claimed in claim 5 and in which the resistivecomponent in the charging circuit is adjustable, to effect compensationfor manufacturing variations in the circuit components.

8. A firing circuit as claimed in claim in which said voltage-responsivecomponent is a unijunction transistor, having an emitter, base one andbase two connections, means for coupling the unijunction transistoremitter to a point in said charging circuit to fire the unijunctiontransistor when the voltage accumulated across the capacitor reaches apredetermined value, and means operative responsive to firing of theunijunction transistor to couple a firing pulse to the firstsemiconductor switchin the switching circuit.

9. In a system for energizing a DC motor from an input circuit havingthree conductors over which three-phase AC energy is received, a powercircuit having at least three SCRs, each with anode, cathode and gateconnections for selective control of SCR conduction time by applicationof an appropriate anode-cathode potential and a gate drive signal, acontrol amplifier stage connected to sense motor terminal voltage andmotor armature current and to provide an output signal indicating thedesired conduction time of the SCRs, and three separate firing circuitsfor the respective SCRs, each firing circuit having an input circuitconnected to receive the output signal from the control amplifier stageand having an additional input circuit for receiving a reset signal,each firing circuit comprising:

an output transistor coupled between -a conductor. for

receiving energy and an output resistor, such that conduction of theoutput transistor develops a gate drive signal across said resistor forapplication between the gate and cathode of the SCR associated with thefiring circuit,

a multivibrator switching circuit having first and second transistorsintercoupled in a flip-flop circuit, such that turn-on of the firsttransistor is effective both to turn off the second transistor and togate on the output transistor, and turn-on of the second transistor iseffective both to turn off the first transistor and to disable theoutput transistor to remove the gate drive signal,

a charging circuit, including a capacitor, the collectoremitter path ofa control transistor, and at least one resistive component, all coupledin series for receiving an energizing potential such that the rate ofcharge of the capacitor is a function of the conduction level of thecontrol transistor,

circuit means, including a unijunction transistor, coupled between thecharging circuit and the multivibrator switching circuit, operative inresponse to accumulation of a predetermined voltage level across thecapacitor to fire the unijunction transistor and pass a pulse signal tothe multivibrator circuit to gate on the first transistor and switchoffthe second transistor, thus turning on the output transistor to supplycontinuous gate drive to the associated SCR, and

a reset circuit, including a reset transistor coupled to said capacitorand to said second transistor in the multivibrator switching circuit,operative responsive to receipt of a reset signal to render the resettransistor conductive to both discharge the capacitor and gate on thesecond transistor in the multivibrator circuit, returning themultivibrator switching circuit to its original state and removing thecontinuous gate drive signal from the associated SCR.

10. A motor-energizing system as claimed in claim 9 and furthercomprising a three-phase transformer having three primary windings andthree secondary windings, means for coupling the primary windings to thethree input conductors over which AC input energy is received, and atleast three separate conductors coupled between said secondary windingsandthe respective reset transistor of each firing circuit, tosynchronize turnoff of the continuous gate drive with the appropriatepolarity reversals of the received AC energy.

11. A motor-energizing system as claimed in claim 10 and furthercomprising a rectifier circuit, coupled between said secondary windingsand the transistor output switch, for supplying the energy to the outputtransistor of each firing circuit for providing the continuous gatedrive signal as the output transistor is gated on.

1. A firing circuit for an SCR having an anode, a cathode, and a gate,comprising: a semiconductor output switch connected to provide acontinuous gate drive signal to the SCR gate when said semiconductoroutput switch is conducting, a switching circuit connected to enable anddisable the semiconductor output switch to provide and remove thecontinuous gate drive signal, a charging circuit, including a capacitorand a variable semiconductor switch, connected such that the capacitoris charged at a rate which is a function of the conduction level of thevariable semiconductor switch, means connected to operate in response toa predetermined voltage appearing across the capacitor to apply atrigger signal to the switching circuit and thus provide the continuousgate drive signal, and reset means connected to effect rapid dischargeof the capacitor, and to disable the semiconductor output switch andremove the gate drive signal.
 2. A firing circuit as claimed in claim 1and in which said switching circuit comprises first and secondsemiconductor switches intercoupled in a multivibrator circuit,operative to change the state of the semiconductor output switch fromnonconducting to conducting each time the trigger signal is supplied tothe switching circuit.
 3. A firing circuit as claimed in claim 1 inwhich said charging circuit includes a potentiometer, adjustable tocompensate for variations in the components of the firing circuit.
 4. Afiring circuit as claimed in claim 1 in which the means which operatesin response to appearance of the predetermined voltage across thecapacitor is a unijunction transistor, and circuit means is coupledbetween the unijunction transistor and the switching circuit for passingthe trigger signal to actuate the switching circuit each time theunijunction transistor is fired.
 5. A firing circuit for an SCR havingan anode, a cathode, and a gate, comprising: a semiconductor outputswitch connected to provide a continuous gate drive signal to the SCRgate when said semiconductor output switch is conducting, a switchingcircuit having first and second semiconductor switches, connected suchthat conduction of the second semiconductor switch disables thesemiconductor output switch and removes the gate drive signal, andconduction of the first semiconductor switch turns off the secondsemiconductor switch and turns on the semiconductor output switch toprovide the continuous gate drive signal, a charging circuit, includinga capacitor, a variable semiconductor switch, and a resistive component,all coupled in seRies with a conductor for energizing the chargingcircuit such that the capacitor is charged at a rate which is a functionof the conduction level of the variable semiconductor switch, means,including a voltage-responsive component, coupled between the chargingcircuit and the switching circuit, for operating in response to apredetermined voltage appearing across the capacitor to apply a triggersignal for turning on the first semiconductor switch in the switchingcircuit and thus providing the continuous gate drive signal, and means,including a reset semiconductor switch, coupled between a conductor forreceiving a reset signal and the capacitor in the charging circuit, foreffecting rapid discharge of said capacitor and conduction of the secondsemiconductor switch in the switching circuit to disable thesemiconductor output switch and remove the gate drive signal.
 6. Afiring circuit as claimed in claim 5 and in which said semiconductoroutput switch is a transistor coupled between a plane of energizingpotential and a plane of reference potential, and a component is coupledbetween the transistor and the reference potential plane to provide thecontinuous gate drive signal whenever the transistor is gated on by asignal from the switching circuit.
 7. A firing circuit as claimed inclaim 5 and in which the resistive component in the charging circuit isadjustable, to effect compensation for manufacturing variations in thecircuit components.
 8. A firing circuit as claimed in claim 5 in whichsaid voltage-responsive component is a unijunction transistor, having anemitter, base one and base two connections, means for coupling theunijunction transistor emitter to a point in said charging circuit tofire the unijunction transistor when the voltage accumulated across thecapacitor reaches a predetermined value, and means operative responsiveto firing of the unijunction transistor to couple a firing pulse to thefirst semiconductor switch in the switching circuit.
 9. In a system forenergizing a DC motor from an input circuit having three conductors overwhich three-phase AC energy is received, a power circuit having at leastthree SCR''s, each with anode, cathode and gate connections forselective control of SCR conduction time by application of anappropriate anode-cathode potential and a gate drive signal, a controlamplifier stage connected to sense motor terminal voltage and motorarmature current and to provide an output signal indicating the desiredconduction time of the SCR''s, and three separate firing circuits forthe respective SCR''s, each firing circuit having an input circuitconnected to receive the output signal from the control amplifier stageand having an additional input circuit for receiving a reset signal,each firing circuit comprising: an output transistor coupled between aconductor for receiving energy and an output resistor, such thatconduction of the output transistor develops a gate drive signal acrosssaid resistor for application between the gate and cathode of the SCRassociated with the firing circuit, a multivibrator switching circuithaving first and second transistors intercoupled in a flip-flop circuit,such that turn-on of the first transistor is effective both to turn offthe second transistor and to gate on the output transistor, and turn-onof the second transistor is effective both to turn off the firsttransistor and to disable the output transistor to remove the gate drivesignal, a charging circuit, including a capacitor, the collector-emitterpath of a control transistor, and at least one resistive component, allcoupled in series for receiving an energizing potential such that therate of charge of the capacitor is a function of the conduction level ofthe control transistor, circuit means, including a unijunctiontransistor, coupled between the charging circuit and the multivibratorswitching circuit, operative in response to accumulation of apredetermined voltage level Across the capacitor to fire the unijunctiontransistor and pass a pulse signal to the multivibrator circuit to gateon the first transistor and switch off the second transistor, thusturning on the output transistor to supply continuous gate drive to theassociated SCR, and a reset circuit, including a reset transistorcoupled to said capacitor and to said second transistor in themultivibrator switching circuit, operative responsive to receipt of areset signal to render the reset transistor conductive to both dischargethe capacitor and gate on the second transistor in the multivibratorcircuit, returning the multivibrator switching circuit to its originalstate and removing the continuous gate drive signal from the associatedSCR.
 10. A motor-energizing system as claimed in claim 9 and furthercomprising a three-phase transformer having three primary windings andthree secondary windings, means for coupling the primary windings to thethree input conductors over which AC input energy is received, and atleast three separate conductors coupled between said secondary windingsand the respective reset transistor of each firing circuit, tosynchronize turnoff of the continuous gate drive with the appropriatepolarity reversals of the received AC energy.
 11. A motor-energizingsystem as claimed in claim 10 and further comprising a rectifiercircuit, coupled between said secondary windings and the transistoroutput switch, for supplying the energy to the output transistor of eachfiring circuit for providing the continuous gate drive signal as theoutput transistor is gated on.